As a rocket, missile, aircraft or another airborne craft moves through the air, electrical charges build up on the outer surface of the body. There are also times when the surface of the airborne craft is subjected to high energy electrical discharge, such as when it flies through an electrical storm or is struck by lightning. When the skin is of metal, typically aluminum, charge dispersal is not a problem. However, airborne craft have increasingly been made using composite materials instead of aluminum to form their outer surface or skin. Composites, which often incorporate fibrous materials in their composition, exhibit high strength to weight ratios but do not normally have the ability to dissipate electrical charges effectively.
To remedy this problem it is necessary to modify the composite structure in a manner which allows for dissipation of electrical energy buildup. One way of doing so is to apply an electrically conductive material directly to one of the fibrous mats or fabrics to be used to make the composite material, thus forming a thin metallic film on the fibers to disperse or dissipate electrical energy. The fibrous mat or fabric to which the electrically conductive material has been applied is used to form the outer surface of the composite structure. The effectiveness of the electrically conductive film is increased if it is continuous and of constant thickness throughout. However, producing a continuous, constant thickness electrically conductive film on flexible fibrous mats or fabrics has proven difficult because of the tendency to break the metallic bonds between adjacent fibers when the fibrous mat or fabric is flexed during subsequent manufacturing operations.
Another method used to create a composite structure with an electrically conductive outer surface is to deposit an electrically conductive material directly onto the outer surface of the composite structure. Although an electrically conductive film is formed, it is frequently discontinuous in places where the electrically conductive material does not adhere to the composite surface; this usually occurs due to a mismatch of adhesion characteristics between the metal and composite polymer surface. Also, where the electrically conductive material adheres, the thickness of the electrically conductive layer tends to vary greatly over the entire surface.
A further method for creating an electrically conductive surface on a composite structure is through the application of a coated polymer film, such as that sold under the trademark Kapton by E. I. DuPont de Nemours & Co., of Wilmington, Delaware. The polymer film has its outer surface coated with a layer of an electrically conductive material to form an electrically conductive film of a desired thickness. The coated film is then bonded to the surface of the composite structure. However, in some environments and with some composite structures, the polymer layer exhibits poor adhesion characteristics allowing the electrically conductive film to peel away from the composite structure. This may be due in part to differences in the coefficients of thermal expansion between the polymer film and the materials from which the composite structures are made.